Serum inhibits rituximab-induced NK-cell CD16 down-modulation and CD54 up-regulation. PBMCs and Raji, Daudi, or FL cells were mixed at a 1:1 ratio for 20 hours in the presence or absence of 50% serum with varying concentrations of rituximab. Surface marker expression was determined using flow cytometry with gating on CD3⁻, CD56⁺ lymphocytes. (A) NK-cell CD16, expressed as median fluorescence, in the absence and presence of serum (n = 3). (B) NK-cell CD54, expressed as a percentage of CD54 bright, in the absence and presence of serum (n = 3). Error bars represent the standard deviation (SD) of the mean.